Conventionally, an electronic photographic image forming apparatus (printing device) such as a copying device, printer, fax machine or the like, is provided with a photosensitive drum, charging device, exposing device, developing device, transfer device, fixing device, and the like.
In such an image forming apparatus, an electrostatic latent image is formed on a photosensitive surface (electrostatic latent image surface) of the photosensitive drum by electrifying the photosensitive surface by the charging device and then exposing the photosensitive surface to light by the exposing device. Then the thus formed electrostatic latent image is developed with toner (developer) by the developing device, thereby to form a toner image (visible image). The toner image is then transferred onto a sheet (recording medium; a printing medium such as plain paper, OHP sheet, or the like) by the transfer device. After that, the toner image is fixed on the sheet by the fixing device.
Moreover, in general, the developing device provided to the image forming apparatus is provided with a developing roller and a toner tank for storing the toner therein. Rotation of the developing roller delivers the toner on the surface of the photosensitive drum sequentially (continuously).
In delivering the toner onto the surface of the photosensitive drum by the rotation of the developing roller as described above, in a downstream of a nip portion between the developing roller and the photosensitive drum, the toner is sometimes scattered from a gap portion between the developer tank and the developing roller. The toner thus scattered would be possibly attracted to the photosensitive drum and cause quality deterioration of the image to be transferred onto the sheet. The scattering of toner is especially easy to occur on such locations that correspond to ends of the developing roller with respect to a direction of a rotation axis thereof.
Therefore, for example, as illustrated in FIG. 4, a conventional developing device is provided with seals 103 respectively in gap portions between ends of the developing roller 101 with respect to the direction of the rotation axis thereof, and a developer tank 102. The seals 103 are provided in a downstream of a nip portion between the developing roller 101 and the photosensitive drum, and made, for example, of urethane sheet (elastic film) or the like. This arrangement aims to prevent the toner from scattering.
Moreover, for example, Japanese Unexamined Patent Application Publication, Tokukaihei, No. 4-248579 (published on Sep. 4, 1992) discloses an art in which 3Bg>G2>Bg for use of a two-component developer which is prepared by mixing toner and carrier together, where G2 is abase gap between a magnet roll (developing roller) and a developer tank, the base gap being located at an introducing section for introducing the developer into the developer tank, and Bg is a blade gap between the magnet roll and a doctor blade. This art reduces the toner scattering.
However, in the arrangement in which the seals are provided, the seal member is so provided that the seals slide on the developing roller. Therefore, it is inevitable that the seals are deteriorated over age, thereby losing their effect of preventing the toner scattering. Moreover, if the seal member comes off or is bent away, the toner scattering cannot be prevented sufficiently.
Further, if the seals are provided only for the ends of the developing roller with respect to the rotation axis thereof, a direction of air flow between the ends which conventionally causes the toner scattering is merely changed inwardly (i.e., inwardly with respect to the direction of the rotation axis) where the seals are provided. In some cases, such an arrangement is not sufficient to prevent the toner scattering.
Moreover, the art of Japanese Unexamined Patent Application Publication, Tokukaihei, No. 4-248579 is relatively effective to reduce the toner scattering when process speed is slow. But, when the process speed is fast, this art cannot prevent the toner scattering.
This is because the rotation of the developing roller causes swirling of the air in the whole region between the magnetic roller and the developer tank at the introducing section for the developer tank, and the swirling air cannot escape out of the developer tank thereby increasing an internal pressure of the developer tank, and causing a flow of air emitting from an opening in a downstream of the rotation of the developing roller. Furthermore, the narrow base gap G2 between the magnetic roller and the developer tank also increases a flow rate in a region around the narrow base gap. This causes synergic effect with the above phenomenon. The synergic effect causes the toner scattering when the process speed is fast. Therefore, in the art of Japanese Unexamined Patent Application Publication, Tokukaihei, No. 4-248579, a faster speed (process speed) causes a greater internal pressure in the developer tank, thereby making it easy for the toner scattering to occur.